Rotational viscometers measure fluid shear stress created by known shear rates. Viscosity is defined as the ratio of shear stress to shear rate, commonly expressed in units of Poise. The shear stress is proportional to the torque induced on a cylindrical object immersed in a fluid surrounded by a concentric cylindrical surface that is rotated at known speeds. The shear rate is a function of the rotational speed and the geometry.
One type of rotational viscometer is a couette viscometer. In a couette viscometer, liquid whose viscosity is to be measured fills a space between two vertical coaxial cylinders. The inner cylinder is suspended while the outer cylinder is rotated at a constant rate using a computer controlled stepper motor. The resulting torque on the inner cylinder is accurately measured to determine shear stress.
Couette viscometers may be used to determine Theological properties of Newtonian or non-Newtonian solid-laden or non-solid-laden fluids used as completion or treatment fluids in oil and/or gas wells. It is desirable to test such fluids at high temperatures and high pressures.
In a typical instrument a pressure vessel capable of withstanding high temperatures and pressures is provided to contain the cup and bob of the viscometer. After testing is conducted, the pressure vessel must be removed from the instrument to change the sample fluid and to clean the components.
To withstand high temperatures and pressures, the pressure vessel is constructed of heavy materials and, is therefore, difficult to remove and inconvenient to handle and manipulate. It is therefore desirable to provide a viscometer for measuring high pressure and high temperature samples that has a construction to facilitate ease of operation and cleaning.